For purposes of clarification and understanding of this document, the following definitions are set forth. The term “wastewater” designates contaminated domestic wastewater such as “sanitary sewage” as well as water that carries away other waste matters from households, mercantile, commercial, and industrial establishments. “Stormwater” refers to rainfall runoff waters collected in sewers. “Extraneous inflows” refer to waters that enter a sewer collection system by infiltration from ground waters and by inflow from surface water sources especially during stormwater runoff periods. “Combined sewer systems” carry “combined sewage” that is a mixture of wastewater and stormwater plus a certain amount of extraneous inflows. In separate wastewater sewer systems, “wastewater mixtures” are the result of wastewater mixing with extraneous inflows.
Combined sewer systems are designed to transport combined sewage in the form of wastewater plus a much larger allowance in flow capacity for stormwater and a minor allowance for extraneous inflow. Combined sewage is commonly collected in municipal combined sewer systems; and during low flow periods, combined sewage is mainly comprised of wastewater which is directed via an interceptor pipeline to a wastewater treatment plant. However, when heavy stormwater flows occur, combined sewer flows often exceed the flow capacity of the interceptor; and the overflow is diverted into a receiving watercourse such as a natural stream or river. These overflows are referred to as combined sewer overflow (“CSO”). However, in high rainfall climates over an annual period, CSOs may be a frequent occurrence resulting in receiving water pollution.
Separate wastewater sewer systems are designed to transport wastewater plus a limited allowance for extraneous inflow. They do not have direct stormwater connections to surface water inlets as do combined sewers. Although separate wastewater sewer systems are intended to carry mainly domestic wastewater, during high rainfall runoff periods, they also may become overloaded due to extraneous inflows and therefore, carry a wastewater mixture, which is usually a more concentrated form of combined sewage. Overflows from separate wastewater or sanitary sewer systems are generally referred to as a “Sanitary Sewer Overflow” (“SSO”). SSOs generally contain a larger wastewater or sanitary sewage component and are of higher pollution concentrations than CSOs from combined sewers.
There is a continuing effort to reduce water pollution in watercourses, streams, rivers, lakes, oceans, and other receiving bodies of water from CSO, which is emphasized in the U.S. Environmental Protection Agency (EPA) guidance documents including “Combined Sewer Overflow (CSO) Control Policy (66 FR 42226) and “Coordinating CSO Long-term Planning with Water Quality Standards Reviews (EPA-833-R-01-002; July 2001)”.
Therefore, there is a need during heavy stormwater runoff periods to prevent, or to some substantial degree avoid, the creation of combined or mixed sewage, thereby reducing CSO and SSO pollution concentrations.